The invention relates to a method and apparatus for the production of cement, and more particularly for the production of raw material to the clinker stage by thermal treatment of the raw material in specific steps including a preheating step, a calcination step which is divided into two stages for a precalcination and after-calcination stage, a sintering step and a cooling step and wherein heat is supplied by combustible fuel in the sintering step and the after-calcination step with the material, air, and gases being uniquely handled for improved process steps and improved resultant material.
Various methods and apparatus for the thermal treatment of raw material to be used in the manufacture of cement have been used heretofore using combustibe fuel for supplying heat in a calcination step and in a sintering step to receive the raw material from the calcination step and supply it to the sintering step in series arrangement. Generally in previous arrangements heretofore used, the sintering step receives combustion air directly from the cooling step and certain arrangements have been used for handling the gas and air from the sintering and cooling step to the calcination step.
In one arrangement, the necessary air for the combustion of fuel is supplied separately to the calcination step being received from the cooling step and may flow in excess through the sintering step and enter in mixture with the exhaust gases of the sintering step into the calcination step. The total quantity of oxygen required in the combustion zone of the calcination step will be advanced with the exhaust gases of the rotary kiln. (German Laid Open Specification No. 23 24 565).
It is also known to supply directly the necessary combustion air for the calcination step directly thereto while bypassing the sintering step. In this arrangement, the air is unmixed for the combustion of fuels in the calcination step, and the exhaust gases are then combined with the exhaust gases of the sintering step passed onto the calcination step and supplied to the preheating step (German Laid Open Specification No. 22 48 030).
Furthermore, it has been known in one form to provide the necessary combustion air the calcination step supplied directly to the calcination step while bypassing the sintering step, however, after mixture with the exhaust gases issuing from the sintering step. This mixture of combustion air and exhaust gases of the sintering step serves for the combustion of fuels delivered to the calcination step, after which the exhaust gases of the calcination step are conveyed entirely to the preheating step (German Laid Open Specification No. 23 60 580).
All of the above methods and methods heretofore used have appreciable disadvantages concerning the procedure of calcination with respect to the speed of operation and heat requirement. A number of these disadvantages reside in the mechanism for driving out the CO.sub.2 from a particle of limestone.
It is known that the calcination of a particle of limestone is dependent on three parameters:
1. The grain size of the particle. PA0 2. the duration of the particle in the atmosphere of the heating medium, and PA0 3. the calcination temperature.
The grain size and grain distribution of an industrially manufactured comminuted raw material for cement can hardly be influenced, and is not controllable through agglomeration procedures in the heat exchanger. The duration of the particles of limestone or calcite in the calcination step is not more than several seconds, for example, in the case of a suspension heat exchanger.
As an economically feasible measure, attempts are, therefore, undertaken in the alteration of the parameter of "calcination temperature" for the following reasons:
From the heating medium which is usually gaseous, which surrounds the particle of limestone, the necessary deacidification heat must be transferred to the particle. For this, of course, the heating medium must necessarily have a higher temperature than the particle of limestone. As soon as the outer layer of the particle is deacidified, deacidification heat must be transferred through the outer layer to the interior of the particle. Both the heat transfer and heat conduction processes proceed more quickly with higher temperature differences. Heat transfer and heat conduction into the center of the particle on the other hand are hindered by the CO.sub.2 being driven out, which flows within the particle opposite the direction of heat transfer, and upon leaving the particle surface, this disturbs the heat transfer from the heating medium to the surface of the particle.
When the heat imparting medium during deacidification is supplied with no further heat, and the heat capacity utilizable for deacidification is determined and limited only through the difference between its temperature at the beginning of the deacidification operation and the deacidification temperature itself, because of the endothermic deacidification operation, the temperature of the heat medium rapidly drops. Therefore, the conditions for the progress of the calcination operation become progressively worse.
If, however, the atmosphere surrounding the particle to be deacidified is constituted as a heat imparting medium so that fuel may be burned, the course of calcination is itself improved because the temperature of the heat imparting medium even upon continuous deacidification of the particle does not drop as long as fuel heat is still supplied. Contrarily, the temperature actually rises theoretically with increasing deacidification and the conduction of heat into the interior of the particle is hindered.
It has been found, however, in practice that this procedure is disturbed when the atmosphere surrounding the particle at the beginning of combustion consists to an appreciable extent of inert gases, even when the gases with higher starting temperatures offer utilizable heat for deacidification. On the other hand, as the specific quantity of gases increase, it leads to the fact that even with the combustion of a predetermined equally large quantity of fuel and pure air, the drop in temperature of the heating medium can attain the lower limit at which no sufficient transfer of heat takes place for deacidification. The supply of further fuel heat for the increase of calcination could indeed theoretically bring about an improvement in the calcination operation. However, this leads to increased losses in exhaust gas at the beginning of the preheating step so that this measure is economically not feasible.
Known methods referred to above incur the disadvantages described, and these occur in one or another manner and lead to an increased use of heat for the calcination of cement clinker. The disadvantages are additionally increased in that the time limit inherent through the use of the available apparatus, for the calcination operation, and because of the non-influenceable grain size and grain distribution, lead to the fact that the heating medium cannot given off the theoretically available quantity of heat.
An object of the present invention is to provide a method and apparatus to overcome the foregoing disadvantages and provide an improved method for the multi-step calcination of cement clinkers to make it possible to carry out an umimpeded and rapid calcination of raw material in the calcination step and at the same time improve the economy of heat utilization in the calcination process.
A feature of the invention is the solution of the problems provided by the prior art, and the attainment of the objectives set forth above by providing the calcination step to be divided into a precalcination stage and an after-calcination stage wherein the raw material to be treated first passes through a precalcination step and subsequently through an after-calcination step. In the precalcination step, the material is heated exclusively by exhaust gases from the sintering kiln and in the after-calcination step the material is heated through the combustion of fuel and by means of air. The division of the calcination of raw material into two stages has the advantage that the calcination procedure is controlled exactly and may be adjusted better to a predetermined degree of calcination. Thus, in the precalcination stage, the heat transfer for high heating of raw material and the beginning of calcination takes place without resistance so that in spite of dropping temperature, the heat imparting medium transfers heat to the materials rapidly. In the after-calcination step, by virtue of the supply of heat from fuel combusted air, it insures that an atmosphere exists advantageous to calcination, and the temperature difference required for a positive heat transfer with continuous calcination can be optimally adjusted. Accordingly, the best possible utilization of fuel is insured with optimum guidance of the calcination process, and the economy of heat of the calcination and combustion process is improved. The arrangement also provides for the utilization of combustion air in the after-calcination step received from exhaust air from the cooling step.
In accordance with the principles of the invention, the exhaust gases of the precalcination and after-calcination stages are used in common for preheating the raw material in the preheating step. In this manner the total quantity of hot exhaust gases from the sintering step and the total quantity of hot combustion gases from the calcination step are utilized for preheating the raw material. In the preheating step, no substantial chemical processes, particularly no release of CO.sub.2 from the limestone takes place, and the utilizable heat content of the hot gases for preheating the raw material are utilized.
It is a further feature of the invention that it is provided that a portion of the exhaust gases from the sintering kiln are bled off before the gases flow to the precalcination step. This arrangement provides that with alkali containing raw materials, the volatilized alkalis may be partially removed from the process so that alkali circulations in the combustion process are reduced to an appreciable degree thereby preventing the danger of disturbances of the calcination process.
The invention also relates to an improved apparatus for carrying out the method of the invention. The apparatus includes a device for preheating, calcination, sintering and cooling of raw material. The device is divided into apparatus for steps and stages and provides a precalcination and after-calcination device wherein the raw material is consecutively passed from one to another, and wherein the precalcination device is used in combination with the sintering kiln, and the after-calcination device in combination with mechanism for the supply of combustion air. Apparatus is furnished for the manufacture of cement to the clinker stage were it is possible to the first time to calcine raw materials optimally in two steps separated from one another. In the precalcination step, the raw material is brought into contact exclusively with hot gases of the sintering step so that there is a very rapid heat transfer takes place to the material. Subsequently, the highly heated and precalcinated material is brought into a hot gas atmosphere in which optimum conditions are provided for the further and complete calcination of the material. Therefore, the calcination step permits the limestone particles to be calcined with a maximum or full utilization of combustion heat. The after-calcination device is used in connection with a cooling device so that the exahsut air of the cooling step is used in the after-calcination step.
The apparatus is arranged to provide that the precalcination and after-calcination devices are used in connection with the preheating step. With this arrangement, it is possible to make use of the entire quantity of hot exhaust gases received in both the precalcination and after-calcination steps for the preheating of the raw material.
In accordance with the invention, the preferred arrangement utilizes cyclone heat exchangers for the preheating, precalcination and after-calcination steps. In the apparatus, it is possible for the comminuted raw materials to be used advantageously in the production of cement, and with fine dispersion of comminuted raw materials in a hot gas stream, particularly favorable conditions are insured with respect to the operation in both precalcination and after-calcination steps.
Other objects, advantages and features, as well as equivalent structures and methods which are intended to be covered herein, will become more apparent with the teaching of the preferred embodiment in the specification, claims and drawings, in which: